Structure and properties of isotactic polypropylene modified with siloxane-silsesquioxane resin and sorbitol derivatives

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POZNAN UNIVERSITY OF TECHNOLOGY INSTITUTE OF MATERIALS TECHNOLOGY Polymer Division

Monika Dobrzyoska-Mizera

Structure and properties of isotactic polypropylene

modified with siloxane-silsesquioxane resin and sorbitol derivatives

The presented dissertation is a guide throughout monothematic series of scientific articles realized under supervision of Prof. Tomasz Sterzyński and Dr. Maria Laura Di Lorenzo.

POZNAO, JUNE 2017

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1

Acknowledgements

While my name may be alone on the front cover of this thesis, I am by no means its sole contributor. Rather, there is a number of people behind this piece of work who deserve to be both acknowledged and thanked here: committed supervisors, kind colleagues, my fantastically supportive husband and marvelous parents who lead me here.

I am forever indebted to my academic supervisors, Prof. Tomasz Sterzyoski and Dr. Maria Laura Di Lorenzo for their enthusiasm, guidance, and unrelenting support throughout this process. They have routinely gone beyond their duties to fire fight my worries, concerns and anxieties, and have worked to instill great confidence in both myself and my work. This piece of research looks very different because of their input, influence and expert knowledge.

I would like to thank my beloved husband Marcin for his unremitting encouragement. Put simply, I have never met anyone who believes in me more. Thank you for making me more than I am.

I am grateful to my parents for everything. You made me into who I am.

Also I wish to show extensive gratitude to all colleagues from Polymer Division at Poznan University of Technology who supported me with their knowledge and kindness from the first day we met.

Lastly, I thank all of the amazing people from Instituto per i Polimeri, Compsiti e

Biomateriali in Pozzuoli who always made me feel in Italy like at home.

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2

Abstract

This study details new polypropylene-based materials containing a siloxane-silsesquioxane resin functionalized with phenyl groups and two types of sorbitol derivatives, namely 1,3:2,4- bis(3,4-dimethylbenzylidene)sorbitol, Millad 3988 (DMDBS), and 1,2,3-tridesoxy-4,6:5,7-bis-O-[(4- propylphenyl)methylene]nonitol sorbitol, Millad NX8000 (NX8000). The aim is to investigate the influence of such additives on the crystallization behavior and morphology, as well as on thermal and mechanical properties of isotactic polypropylene nucleated with the modified sorbitols, as a function of the molecular characteristics of both the sorbitol derivative and phenyl siloxane- silsesquioxane resin. The goal is to develop an iPP-based formulation which crystallization kinetics is tailored to extrusion processing, in order to optimize material properties without enhancement of production costs.

Composites based on isotactic polypropylene (iPP) modified with the sorbitol derivative NX8000 and siloxane-silsesquioxane resin containing reactive phenyl groups (SiOPh) were prepared by melt extrusion. The addition of sorbitol fastens crystallization kinetics of iPP and leads to higher transparency of iPP films. Upon the incorporation of siloxane-silsesquioxane resin, no further effect on iPP crystallization kinetics is evidenced by calorimetry, optical microscopy, and X-ray diffraction analysis. Transparency of iPP-based composites is improved upon the addition of sorbitol, but decreased when SiOPh is added to the formulation. The composites are also stiffer, compared to neat polypropylene with a decreased elongation at break and increased Young’s modulus values, with increasing amounts of fillers. The effect of the siloxane- silsesquioxane resin on properties of iPP/NX8000/SiOPh composites was explained taking into account compatibility of the components and morphology of the composites.

Better results were obtained upon formulation of iPP with another sorbitol derivative, namely DMDBS, plus SiOPh. Compared to NX8000, DMDSB has varied functionalization of the hydroxyl rings, which may affect interaction with SiOPh. Molecular adducts originating from the synergistic interactions of siloxane-silsesquioxane resin (SiOPh) and Millad 3988 (DMDBS), influence crystallization kinetics of isotactic polypropylene (iPP), as well as its spherulitic morphology, transparency and mechanical properties. DMDBS is a commonly used clarifying agent for a variety of iPP grades. However, its disadvantage is that when added into molten iPP it allows only low draw ratios because it increases crystallization temperature of iPP. Addition of SiOPh allows to control the nucleation efficiency of the sorbitol derivative and adjust crystallization rate of iPP, to attain transparent formulations suitable for extrusion processes.

All presented iPP-based compounds were produced by co-rotating twin screw extrusion and further analyzed by differential scanning calorimetry, wide-angle X-ray diffraction, scanning electron microscopy and Fourier transform infrared spectroscopy. Moreover, rheological, haze and static tensile measurements were conducted to determine the influence of composition on material properties. It was found that the best combination of properties is achieved when 1 wt%

of SiOPh and 0.25 wt% DMDBS are added to iPP. This formulation can significantly change the crystallization behavior of iPP to be tailored for production of highly oriented and transparent products.

Further improvements were attained by addition of a compatibilizer. iPP composites

modified with NX8000 sorbitol derivative and siloxane-silsesquioxane resin (SiOPh) containing

maleated polypropylene (MAPP) as compatibilizer were prepared in order to favor interaction

between fillers. Calorimetric investigations revealed no influence of SiOPh and a slight effect of

MAPP addition on the crystallization kinetics of polypropylene. Additionally, the introduction of

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3

Abstract

MAPP into the iPP+NX8000+SiOPh composites increased elongation at break of the samples. All the above was attributed to the compatibilizing effect of MAPP which improved interfacial adhesion between iPP, NX8000 and SiOPh. This phenomenon was also confirmed by the SEM images illustrating more homogenous distribution of the filler in the compatibilized samples.

Based on the above research, formulations of iPP-based composites were tailored by

incorporation of a sorbitol derivative and a compatibilizer. Functional groups of MAPP may

suppress fibrillation of sorbitol derivative upon cooling of polypropylene and result in a control of

crystallization kinetics in iPP-based composites during production. Moreover, MAPP is

commercially available and relatively cheap in use. The addition of MAPP strongly influences

crystallization of isotactic polypropylene (iPP) as evidenced via differential scanning calorimetry

measurements. Changes in rheological properties and transparency were also examined and

turned out to be improved by the proposed modification.

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4

Streszczenie

Izotaktyczny polipropylen (iPP) jest semikrystalicznym i polimorficznym polimerem termoplastycznym o maksymalnym stopniu krystaliczności równym 60-70%. iPP może krystalizowad pod postacią różnych struktur krystalicznych, dlatego różnicując warunki chłodzenia i/lub wprowadzając do osnowy polimerowej środki nukleujące możliwe jest wytworzenie różnych morfologii w osnowie polimerowej. W rezultacie, iPP może formowad struktury zwane α, β, ,  (w obecności komonomerów) oraz



, jak i nieuporządkowaną mezofazę. Dzięki podwyższonej transparentności oraz wyższemu modułowi elastyczności i wytrzymałości na rozciąganie, w porównaniu z pozostałymi formami krystalicznymi, faza α jest jedną z najczęściej wykorzystywanych.

Najczęściej wykorzystywanymi i najbardziej wydajnymi środkami nukleującymi fazę α w izotaktycznym polipropylenie są pochodne sorbitoli. Mimo, że ich koszt jest dośd wysoki wykazują one unikalny wpływ na właściwości optyczne. Stąd są często stosowane jako środki zwiększające transparentnośd (claryfing agents), a żadne taosze odpowiedniki zdolne do podobnej modyfikacji polimeru nie zostały jak dotąd opracowane. Dodatek sorbitoli skutkuje również podwyższoną temperaturą krystalizacji oraz tworzeniem struktury drobno sferolitycznej. Pochodne sorbitoli ulegają rozpuszczeniu w stopionym polipropylenie, co pozwala na ich skuteczne zdyspergowanie w matrycy polimerowej podczas przetwórstwa. Podczas chłodzenia polipropylenu ze stopu, rozpuszczony w osnowie polimerowej sorbitol przechodzi separację fazową formując sied fibryli, która stanowi zarodki krystalizacji dla tworzonych sferolitów polipropylenu. Efekt ten ma istotne znaczenie z aplikacyjnego punktu widzenia, ponieważ początek wzrostu krystalitów w wyższej temperaturze pozwala na skrócenie cyklu przetwórczego, a tym samym do redukcji kosztów produkcji np. w procesie wtryskiwania. Niemniej, zbyt duży wzrost temperatury krystalizacji, a więc temperatury zestalania polimeru może byd wadą w przypadku wytwarzania wyrobów wytłaczanych takich jak, folie czy włókna, ponieważ formowanie struktury następuje przed rozpoczęciem procesu krystalizacji. W konsekwencji, przyspieszony proces krystalizacji może ograniczad możliwośd produkcji wysoce zorientowanych wyrobów w trakcie ich chłodzenia.

W wyniku szerokich studiów literaturowych oraz wstępnych badao własnych sformułowano następującą hipotezę realizowanej rozprawy doktorskiej. „W wyniku oddziaływania grup fenylowych występujących w żywicy siloksanowo-silseskwioksanowej z grupami hydroksylowymi w pochodnych sorbitoli powstają addukty, które mogą wpływad na strukturę polimeru, a w konsekwencji na kinetykę krystalizacji oraz właściwości reologiczne i fizyczne izotaktycznego polipropylenu.”

W ostatnich latach uwaga wielu naukowców skupiała się na tworzeniu hybrydowych materiałów, które następnie aplikowano do matrycy polipropylenowej. Takie materiały, nazywane modyfikowanymi silseskwioksanami (POSS), są połączeniem organicznych komponentów z siloksanowym lub krzemowym szkieletem. Ta klasa hybrydowych materiałów posiada unikatowe możliwości, które łączą właściwości organicznych ugrupowao z zaletami krzemowej matrycy. Co więcej, obecnośd organicznych grup wpływa na zwiększenie hydrofobowości powierzchni cząstki, skutkując potencjalną poprawą kompatybilności między napełniaczem, a matrycą polimerową.

Jednak, na podstawie analizy wyników badao stwierdzono, że te organiczno-nieorganiczne

materiały nie są w pełni kompatybilne z iPP. Stąd wypracowano alternatywne podejście

polegające na połączeniu komercyjnie stosowanych nukleantów z silseskwioksanami. Wykazano,

że cząsteczki silseskwioksanów modyfikowanych grupami fenylowymi, w połączeniu z sorbitolami

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5

Streszczenie

mają możliwośd formowania złożonych adduktów poprzez wiązania wodorowe. Formowanie struktur molekularnych zapobiega tworzeniu się sieci fibryli pochodnej sorbitolu, co ogranicza efektywnośd nukleacji i pomaga kontrolowad proces krystalizacji polipropylenu. Niestety, wysoki koszt syntezy POSS ograniczył możliwośd jego aplikacji, stąd nadal istnieje potrzeba poszukiwania taoszych modyfikatorów.

Nowatorskim pomysłem jest wytworzenie modyfikatora posiadającego w swojej budowie zarówno organiczne jak i nieorganiczne elementy, które mogą reagowad z pochodnymi sorbitolu oraz matrycą polimerową; ich synteza jest prostsza w porównaniu z wcześniej aplikowanymi POSS, przez co taosza w zastosowaniu. Dlatego podjęto starania mające na celu wytworzenie nowego związku, tj. żywicy siloksanowo-silseskwioksanowej modyfikowanej grupami fenylowymi (SiOPh). Zastosowanie takiej żywicy w połączeniu z izotaktycznym polipropylenem nukleowanym sorbitolem nie zostało dotąd opisane w literaturze. Żywica zawierająca grupy fenylowe może oddziaływad z grupami funkcyjnymi sorbitolu, jak również z alkilowymi grupami bocznymi, co może wspomóc kompatybilizację z osnową polimerową.

Celem poznawczym pracy jest określenie wpływu jednoczesnego oddziaływania pochodnych sorbitoli oraz żywicy siloksanowo-silseskwioksanowej na strukturę i właściwości izotaktycznego polipropylenu. Do opisu interakcji występujących między modyfikatorami

zastosowano dwa rodzaje środków nukleujących, mianowicie 1,2,3-tridesoxy-4,6:5,7-bis-O-[(4- propylphenyl)methylene]nonitol sorbitol (NX8000) oraz 1,3:2,4-bis(3,4- dimethylbenzylidene)sorbitol (DMDBS). Związki te zawierają w swojej budowie aktywne grupy funkcyjne, które potencjalnie mogą oddziaływad z grupami bocznymi żywicy siloksanowo- silseskwioksanowej. Jednak ze względu na zróżnicowane rozłożenie przestrzenne grup hydroksylowych możliwe jest tworzenie przez te związki zawad przestrzennych, które z kolei będą wpływały na ich reaktywnośd. Stąd analiza mechanizmu tworzenia indywiduów chemicznych między zastosowanymi w pracy modyfikatorami oraz ich wpływ na właściwości polipropylenu wymagają wprowadzenia do osnowy polimerowej nukleantów o różnej budowie chemicznej.

Plan badao zakładał ocenę wpływu wymienionych addytywów na krystalizację i morfologię, jak również na właściwości termiczne i mechaniczne izotaktycznego polipropylenu. W tym celu wytworzono mieszaniny polipropylenu z dodatkiem 0,25% wag. czynnika nukleującego oraz zawartości żywicy siloksanowo-silseskwioksanowej w zakresie od 0,1 do 3% wag. mieszane wstępnie w mieszadle mechanicznym. Tak przygotowane składniki poddano następnie ujednorodnianiu w procesie wytłaczania. Celem było opracowanie składu matrycy polipropylenowej, tak aby możliwa była kontrola procesu krystalizacji polipropylenu podczas formowania wyrobów na drodze wytłaczania w stanie stopionym, a także optymalizacja właściwości materiału z uwzględnieniem kosztów modyfikatorów.

Analiza wstępnych wyników badao potwierdziła, że jednoczesne wprowadzenie do

izotaktycznego polipropylenu pochodnej sorbitolu DMDBS oraz żywicy siloksanowo-

silseskwioksanowej skutkuje możliwością regulowania procesu porządkowania struktury

krystalicznej osnowy polimerowej. W konsekwencji zmianie ulegają właściwości mechaniczne

izotaktycznego polipropylenu. Możliwe jest zatem stwierdzenie, że modyfikacja polipropylenu

żywicą oraz DMDBS, poprzez oddziaływania chemiczne między addytywami, wpływa na

efektywnośd nukleacji fazy α polipropylenu. Natomiast, badania optyczne wytworzonych

mieszanin potwierdziły, że dodatek SiOPh nie wpływa na pogorszenie transparentności osnowy

polimerowej. Dzięki temu możliwe jest uzyskanie wyrobów o zwiększonej transparentności i

polepszonych właściwościach wytrzymałościowych. Powyższe zostało potwierdzone na podstawie

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Streszczenie

analizy wyników badao statycznej próby rozciągania, gdzie stwierdzono wzrost wartości modułu sprężystości modyfikowanych próbek.

Nieco odmienne rezultaty uzyskano dla próbek modyfikowanych NX8000 oraz SiOPh.

Wprowadzenie NX8000 inicjuje proces porządkowania struktury polipropylenu w wyższych temperaturach, podobnie jak w przypadku DMDBS, jednak na skutek braku oddziaływao między nukleantem a żywicą siloksanowo-silseskwioksanową, nie zaobserwowano efektu „kontrolowanej krystalizacji”, tj. nie odnotowano możliwości sterowania temperaturą początku procesu porządkowania struktury krystalicznej. Stąd, w oparciu o wyniki wstępnych badao własnych opracowano system addytywów, który oprócz NX8000 oraz SiOPh zawiera kompatybilizator w postaci polipropylenu szczepionego z bezwodnikiem maleinowym. Stwierdzono, że takie podejście stwarza możliwośd regulowania procesu tworzenia się specyficznej struktury krystalicznej, co jednoznacznie przekłada się na właściwości użytkowe wyrobów.

W oparciu o studia literaturowe oraz wyniki badao własnych stwierdzono, że zaplanowane pomiary kalorymetryczne i reologiczne oraz wybór mierzonych wielkości, takich jak: stopieo krystaliczności, szybkośd krystalizacji, lepkośd, a także ocena transparentności czy właściwości wytrzymałościowych, pozwoliły na opisanie zależności między charakterystycznymi parametrami a zastosowanymi systemami modyfikującymi i szybkościami chłodzenia polimeru. Badania naukowe w ramach realizowanej pracy doktorskiej prowadzone były w celu:



charakterystyki żywicy siloksanowo-silseskwioksanowej,

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oceny wpływu wymienionych addytywów na krystalizację i morfologię,

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zdefiniowania struktury krystalicznej modyfikowanego polipropylenu,

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analizy właściwości optycznych i mechanicznych matrycy polimerowej modyfikowanej sorbitolami w funkcji zawartości środków nukleujących i żywicy siloksanowo- silseskwioksanowej oraz



oceny właściwości reologicznych wytworzonych mieszanin polimerowych.

Wiązania chemiczne występujące w strukturze żywicy siloksanowo-silseskwioksanowej

opisano za pomocą badao metodą spektroskopii fourierowskiej, wykorzystując spektrometr FTIR

Bruker Tensor 27 pracujący w trybie ATR (Attenuated total reflectance). Natomiast, wielkośd

ziaren proszku żywicy SiOPh zmierzono przy użyciu mikroskopu skaningowego FEI Quanta 200

FEG. Ocena wpływu zastosowanych addytywów na kinetykę krystalizacji oraz morfologię

polipropylenu była możliwa dzięki zastosowaniu skaningowej kalorymetrii różnicowej (DSC) oraz

mikroskopii optycznej (MPO). Wyznaczone przy pomocy skaningowego kalorymetru różnicowego

firmy Mettler Toledo, model DSC 822, krzywe egzo oraz endotermiczne były podstawą opisu

procesów topienia oraz krystalizacji osnowy polimerowej. Ponadto, fotografie struktury oraz

pomiar szybkości wzrostu sferolitów, wykonane za pomocą mikroskopu optycznego firmy Zeiss

wyposażonego w stolik grzewczy Linkam TMHS 600, stanowiły uzupełnienie opisu procesów

topienia i krystalizacji modyfikowanej osnowy polimerowej. Fotografie SEM, wykonane przy

pomocy wyżej opisanego mikroskopu skaningowego, pozwoliły na jednoznaczne określenie

stopnia zdyspergowania modyfikatorów w osnowie polimerowej. Zweryfikowano również wpływ

zastosowanych nukleantów na strukturę izotaktycznego polipropylenu za pomocą pomiarów

prowadzonych przy użyciu dyfraktometru rentgenowskiego firmy Philips, model Analytical X-Ray

PW 1830. W celu oszacowania właściwości optycznych uzyskanych materiałów wykonano pomiary

zamglenia za pomocą urządzenia firmy Murakami Color Research Laboratory, model HM-150. To

badanie umożliwiło ocenę stopnia transparentności badanego tworzywa, a tym samym określid

jego przydatnośd pod kątem zastosowania jako materiału przeznaczonego na folie

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7

Streszczenie

opakowaniowe. Ponadto, wykonano badania statycznej próby rozciągania z wykorzystaniem maszyny wytrzymałościowej Instron, model 4505. Określając podstawowe właściwości wytrzymałościowe uzyskanych kompozycji, takie jak wytrzymałośd na rozciąganie, wydłużenie przy zerwaniu czy moduł Young’a. Wykonano także serię pomiarów reologicznych podczas chłodzenia polimeru ze stopu mających na celu określenie wpływu modyfikatorów na temperaturę, w której następuje gwałtowny wzrost lepkości stopu, co związane jest z początkiem procesu krystalizacji.

Przeprowadzono również analizę oddziaływao między addytywami oraz ich wpływ na strukturę osnowy polimerowej, przy użyciu spektroskopii fourierowskiej mieszanin sorbitolu i żywicy w stosunku 1:1, które pozwoliły na opisanie interakcji zachodzących między nimi. Zmiana intensywności piku pochodzącego od wiązao hydroksylowych jednoznacznie świadczy o reakcji chemicznej zachodzącej między addytywami.

Przeprowadzenie wyżej opisanego toku badao pozwoliło na weryfikację stwierdzenia

stanowiącego, że jednoczesne stosowanie pochodnych sorbitoli oraz żywicy siloksanowo-

silseskwioksanowej modyfikowanej aktywnymi grupami fenylowymi, przyczynia się do

wytwarzania transparentnych wyrobów o polepszonych właściwościach mechanicznych.

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8

Foreword

The presented research was conducted at Poznan University of Technology, Institute of Materials Technology (PUT-IMT) and at Consiglio Nazionale delle Ricerche, Instituto per i Polimeri, Compsiti e Biomateriali, Pozzuoli, Italy (CNR-IPCB), under supervision of Prof. Tomasz Sterzyoski and Dr. Maria Laura Di Lorenzo. Prof. Sterzyoski has been supervising research work of Monika Dobrzyoska-Mizera since she began her PhD studies. The collaboration with Dr. Di Lorenzo started in March 2015, when Monika Dobrzyoska-Mizera spent a 3-month internship at CNR-IPCB, between March and May 2015. This cooperation continued during the whole PhD studies of Monika Dobrzyoska-Mizera, with several additional stages at CNR-IPCB in October 2015, April 2016, September-October 2016, and February 2017. The mutual research interest is focused on development of novel isotactic polypropylene-based formulations with tailored properties. The various stages at CNR-IPCB were supported by both PUT-IMT and CNR-IPCB, as well as by grants awarded to Monika Dobrzyoska-Mizera. She received support via Erasmus+ Programme Staff mobility for training, financed by the European Union, and she was also awarded a CNR-short- term mobility grant to support a two-week research at CNR-IPCB, based on the proposed research and the scientific achievements of Monika Dobrzyoska-Mizera.

The frequent visits of Monika Dobrzyoska-Mizera at CNR-IPCB and the continuation of her research and studies at PUT-IMT, allowed to establish a joint cooperation between the two research institutions, which has led to noteworthy results, published so far in three scientific articles, one patent pending, and six proceedings. Further publications are in preparation.

Preliminary research, dealing with preparation of polypropylene-based composites modified with sorbitol derivatives and silsesquioxanes, was conducted in the frame of the project titled “Silsesquioxanes as fillers and modifiers for polymeric composites”, financed by the European Regional Development Fund within Innovative Economy Programme. In the project, Monika Dobrzyoska-Mizera conducted research on a new type of silsesquioxane functionalized with phenyl groups and proved to be an efficient modifier for a sorbitol derivative. The results have been published in five scientific manuscripts. On this basis, an application of functionalized siloxane-silsesquioxane resin was proposed as it could potentially replace silsesquioxanes due to its low cost synthesis procedure.

The research presented in the thesis was supported by the project “Scholarship support for doctorate students studying at faculties perceived as strategic from Wielkopolska development point of view”, submeasure 8.2.2. of Operational Programme “Human Resources Development”. Italian National Research Council (CNR) Short-Term Mobility Program is also greatly acknowledged. The author was also awarded with Etiuda doctoral scholarship, registered as 2016/20/T/ST8/00399, funded by the National Science Center in Poland.

Milliken & Company and Chemtura are greatly acknowledged for supplying the nucleating

agent and compatibilizer samples for research.

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9

Series of scientific publications

The research articles numbered 1-3 concern modification of isotactic polypropylene with siloxane- silsesquioxane resin and sorbitol derivatives. On the basis of research conducted in Ref. 1-3, a novel polypropylene-based formulation with enhanced mechanical and optical properties was developed and presented in a patent application (4). Preliminary research of iPP-based compositions modified with silsesquioxanes was described in publications numbered 5-9.

1. M. Dobrzyoska-Mizera, M. Dutkiewicz, T. Sterzyoski, M. L. Di Lorenzo, Isotactic polypropylene modified with sorbitol-based derivative and siloxane-silsesquioxane resin, European Polymer Journal 2016, 85, 62-71. (attachment 1)

2. M. Dobrzyoska-Mizera, M. Dutkiewicz, T. Sterzyoski, M. L. Di Lorenzo, Polypropylene- based composites containing sorbitol-based nucleating agent and siloxane-silsesquioxane resin, Journal of Applied Polymer Science 2016, 133, 22. (attachment 2)

3. M. Dobrzyoska-Mizera, M. Dutkiewicz, T. Sterzyoski, M. L. Di Lorenzo, Interfacial enhancement of polypropylene composites modified with sorbitol derivatives and siloxane-silsesquioxane resin, AIP Conference Proceedings 2015, 1695, 020049.

(attachment 3)

4. M. Dobrzyoska-Mizera, T. Sterzyoski, M. L. Di Lorenzo, Kompozyt izotaktycznego polipropylenu o kontrolowanej szybkości krystalizacji i polepszonych właściwościach optycznych oraz sposób jego otrzymywania, patent application no. P.421623.

(attachment 4)

5. M. Barczewski, M. Dobrzyoska-Mizera, B. Dudziec, T. Sterzyoski, Influence of a sorbitol- based nucleating agent modified with silsesquioxanes on the non-isothermal crystallization of isotactic polypropylene, Journal of Applied Polymer Science 2014, 131, 8.

(attachment 5)

6. M. Barczewski, D. Chmielewska, M. Dobrzyoska-Mizera, B. Dudziec, T. Sterzyoski, Thermal stability and flammability of polypropylene-silsesquioxane nanocomposites, International Journal of Polymer Analysis and Characterization 2014, 6, 19. (attachment 6)

7. M. Barczewski, B. Dudziec, M. Dobrzyoska-Mizera, T. Sterzyoski, Synthesis and influence of sodium benzoate silsesquioxane based nucleating agent on thermal and mechanical properties of isotactic polypropylene, Journal of Macromolecular Science, Part A: Pure and Applied Chemistry 2014, 11, 51. (attachment 7)

8. M. Barczewski, M. Dobrzyoska-Mizera, J. Andrzejewski, D. Chmielewska, Ocena właściwości włókien orientowanych wykonanych z nukleowanego izotaktycznego polipropylenu modyfikowanego silseskwioksanami, Przetwórstwo Tworzyw 2013, 3, 153.

(attachment 8)

9. M. Dobrzyoska-Mizera, M. Barczewski, B. Dudziec, T. Sterzyoski, Influence of the cooling

rate on the non-isothermal crystallization of iPP nucleated with DMDBS and

silsesquioxanes, Polimery 2013, 11/12, 58. (attachment 9)

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Series of scientific publications

Proceedings

1. Kompozyty na bazie polipropylenu modyfikowane pochodną sorbitolu i żywicą siloksanowo-silseskwioksanową, XIII Konferencja Naukowo – Techniczna „Kierunki Modyfikacji i Zastosowao Tworzyw Polimerowych”, Rydzyna, 15-17 May 2017.

2. Analiza oddziaływao pochodnych sorbitolu z żywicą siloksanowo-silseskwioksanową na strukturę i właściwości izotaktycznego polipropylenu, VI Konferencja Naukowa, Materiały Polimerowe Pomerania – Plast 2016, Międzyzdroje, 7 – 10 June 2016.

3. Polypropylene-based composites containing sorbitol-based nucleating agent and siloxane- silsesquioxane resin, The 24th Annual World Forum on Advanced Materials PolyChar 24, Poznao, 9-13 May 2016.

4. Polypropylene-based composites containing sorbitol-based nucleating agents and siloxane-silsesquioxane resin, The 14th Young Researchers’ Conference „Materials Science and Engineering”, Belgrade, Serbia, 9-11 December 2015.

5. Interfacial enhancement of polypropylene composites modified with sorbitol derivatives and siloxane-silsesquioxane resin, International conference GT70: Polymer processing with resulting morphology and properties, Salerno, Italy, 15-17 October 2015.

6. Wpływ pochodnej sorbitolu i żywicy siloksanowo-silseskwioksanowej na szybkośd wzrostu sferolitów izotaktycznego polipropylenu, XVIII Profesorskie Warsztaty Naukowe Przetwórstwo Tworzyw Polimerowych, Brodowo k/Środy Wlkp., 2-4 June 2015.

7. Właściwości reologiczne izotaktycznego polipropylenu modyfikowanego sorbitolami i silseskwioksanami, Polski Kongres Reologii, Poznao, 13 – 15 October 2013.

8. Ocena właściwości włókien orientowanych wykonanych z nukleowanego izotaktycznego polipropylenu modyfikowanego polisilseskwioksanami, XII Konferencja Naukowo – Techniczna „Kierunki Modyfikacji i Zastosowao Tworzyw Polimerowych”, Rydzyna, 13-15 May 2013.

9. Ocena skuteczności nukleacji izotaktycznego polipropylenu na podstawie badao skaningowej kalorymetrii różnicowej, IV Międzyuczelniane Seminarium Studenckich Kół Naukowych i Studiów Doktoranckich Inżynieria Wytwarzania, Kalisz, 29-30 October 2012.

Author’s contribution

The author of this dissertation prepared the propylene-based novel materials and carried out

experimental measurements to assess their properties. The research plan, developed by the

author of this dissertation, included several experimental techniques: Differential Scanning

Calorimetry (DSC), Scanning Electron Microscopy (SEM), Optical Microscopy (POM), rheology,

Fourier Transform Infrared Spectroscopy (FTIR), Wide-Angle X-Ray Diffraction (WAXD), haze and

static tensile measurements. Moreover, she conducted literature review, as well as critical

analysis and interpretation of the experimental data. Both the research plan and publications of

the main results were supervised by Prof. Tomasz Sterzyoski and Dr. Maria Laura Di Lorenzo.

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11

List of abbreviations

DMDBS

1,3:2,4-bis(3,4-dimethylbenzylidene)sorbitol

DSC

Differential Scanning Calorimetry

FTIR

Fourier Transform Infrared Spectroscopy

iPP

Isotactic polypropylene

MAPP

polypropylene-graft-maleic anhydride

NX8000

1,2,3-tridesoxy-4,6:5,7-bis-O-[(4-propylphenyl)methylene]nonitol sorbitol

POM

Polarized Optical Microscopy

SiOPh

Siloxane-silsesquioxane resin modified with phenyl groups

SEM

Scanning Electron Microscopy

WAXD

Wide-Angle X-Ray Diffraction

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12

Table of content

Acknowledgements ... 1

Abstract ... 2

Streszczenie ... 4

Foreword ... 8

Series of scientific publications ... 9

Proceedings ... 10

Author’s contribution ... 10

List of abbreviations ... 11

Table of content ... 12

1. Introduction ... 14

2. Experimental ... 16

2.1 Materials ... 16

2.2 Preparation of SiOPh ... 16

2.3 Sample preparation ... 17

2.4 Methodology ... 18

2.4.1 Scanning Electron Microscopy (SEM) ... 18

2.4.2 Differential Scanning Calorimetry (DSC) ... 18

2.4.3 Polarized Optical Microscopy (POM) ... 18

2.4.4 Oscillatory Rheological Measurements ... 19

2.4.5 Fourier Transform Infrared Spectroscopy (FTIR) ... 19

2.4.6 Wide-Angle X-ray Diffraction (WAXD)... 19

2.4.7 Haze ... 19

2.4.8 Static Tensile Tests ... 19

3. Results and discussion ... 20

3.1 Structure and crystallization behavior ... 20

3.2 Rheological properties ... 27

3.3 Intermolecular interactions ... 29

3.4 Crystal phases in polypropylene ... 30

3.5 Performance of iPP-based composites ... 31

3.6 Interfacial enhancement of iPP-based composites ... 35

3.6.1 Structure analysis ... 36

3.6.2 Performance of compatibilized composites ... 37

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13

Table of content

3.7 Commercial iPP-based composites with controlled crystallization rate and enhanced

transparency ... 39

4. Conclusions ...41

5. Literature ...42

Attachment 1...45

Attachment 2...46

Attachment 3...47

Attachment 4...48

Attachment 5...49

Attachment 6...50

Attachment 7...51

Attachment 8...52

Attachment 9...53

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14

Introduction

1. Introduction

Isotactic polypropylene (iPP) is one of the most widely used thermoplastic polymer due to its good mechanical properties, low cost and ease of processing [1-3]. Similar to other semicrystalline polymers, its properties depend on crystal fraction, crystal morphology, as well as on crystal modification. iPP can develop various crystal forms called α-, β-, -, and ε-structures, as well as a conformationally disordered mesophase [4-21]. -crystal modification of polypropylene is industrially significant as it reveals good thermomechanical properties and high impact resistance [5,12,13,21+. However, the α-structure is the most important polymorph from commercial point of view, since it ensures to iPP better transparency as well as higher modulus and tensile strength, compared to the other crystal modifications [24-25]. iPP processing and formulation can be tailored to obtain the crystal structure and morphology, for instance by varying crystallization conditions and/or introduction of nucleating agents into the polymeric matrix.

Sorbitol derivatives are extensively used clarifiers to obtain transparent iPP products due to their high efficiency [22-23, 26]. Although the cost of sorbitol derivatives is relatively high, their effect on polypropylene transparency is unique, not attainable by other cheaper nucleating agents actually available. Moreover, the addition of sorbitol derivatives results in the beginning of polypropylene crystallization at higher temperatures. They melt at processing temperature of polypropylene and form a fibrillar network upon polymer cooling. This creates fine, homogenous dispersion of spherulites with minimalized scattering of light, which is crucial for applications [27,28]. Furthermore, it is an advantage during injection molding, as crystallization at higher temperature allows to shorten the cycle time, thus reduce production cost. Conversely, it is detrimental in case of extrusion, because too intense nucleation limits production of oriented goods, such as films and fibers, since these products can be shaped exclusively prior to crystallization, i.e. before formation of spherulitic structure [29]. Hence, if crystallization occurs at too high temperature, only limited stretching can be applied during extrusion.

For this reason, large research efforts are devoted to sorbitol modifiers capable of controlling crystallization process of nucleated polypropylene. It was proved that the addition of silsesquioxanes (POSS), being cage-like organosilicon compounds functionalized with phenyl side groups, to polypropylene matrix nucleated with a sorbitol derivative allowed to stretch polypropylene with higher draw ratios during extrusion [30-34]. Silanol silsesquioxane and the sorbitol derivative, namely di(benzylidene)sorbitol, are capable of forming several complex molecular adducts, thanks to noncovalent hydrogen bonding. This partially interferes with fibrillation of the sorbitol derivative upon cooling and in turn allows to tailor crystallization of iPP [35]. Unfortunately, the very high cost of commercial POSS has limited its use so far, and there is still a need to seek for lower cost modifiers.

As an effort to find lower cost modifiers able to control crystallization of polypropylene

containing sorbitol derivatives, a novel siloxane-silsesquioxane resin (SiOPh) was synthesized in

our laboratory. Siloxane-silsesquioxane resins can be easily modified with functional groups

similar to POSS and can potentially replace silsesquioxanes thanks to their much lower cost [36-

39]. The siloxane-silsesquioxane resin that we used as additive to iPP nucleated with sorbitol,

contains silanol groups as well as side alkyl groups, as illustrated in Figure 1, below. The silanol

groups are expected to interact with the functional groups of sorbitol [30], and the side alkyl

groups may promote blending with polyolefins [40]. Thus, the addition of SiOPh into nucleated

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15

Introduction

iPP may allow to lower crystallization temperature compared to iPP/sorbitol only, while preserving the clarifying effect gained upon the addition of the sorbitol derivative. This in turn will provide iPP-based products with enhanced transparency and anisotropic mechanical properties.

To our knowledge, SiOPh has never been used as additive to iPP/sorbitol, as no patent was issued/deposited, and no research results have been published in the literature on silimar formulations.

The influence of a newly developed siloxane-silsesquioxane resin with phenyl groups, on properties of iPP containing a commercial sorbitol derivatives, namely 1,3:2,4-bis(3,4- dimethylbenzylidene)sorbitol, Millad 3988 (DMDBS) and 1,2,3-tridesoxy-4,6:5,7-bis-O-[(4- propylphenyl)methylene]nonitol sorbitol (NX8000), are presented in this PhD thesis. Since the modifiers contain free hydroxyl groups, able to create mutual molecular adducts, they are expected to suppress fibrillation of percolated network of sorbitol derivative upon cooling of polypropylene [35]. This allows for tailoring the crystallization process of polypropylene matrix and enables production of transparent, highly oriented films or fibers. Compared to NX8000, DMDSB has varied functionalization of the hydroxyl rings, which may affect interaction with SiOPh. The influence of both additives on the crystallization under static and shearing conditions, as well as morphology, mechanical and optical properties of iPP were investigated.

The aim of the research work detailed in this thesis is to investigate the influence of such additives on the crystallization behavior and morphology, as well as on thermal and mechanical properties of isotactic polypropylene nucleated with the modified sorbitol, as a function of the molecular characteristics of both the sorbitol derivative and phenyl siloxane-silsesquioxane resin. The goal is to develop an iPP-based formulation which crystallization kinetics is tailored to extrusion processing, which is needed to optimize material properties taking into account production costs as well.

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16

Experimental

2. Experimental 2.1 Materials

A commercial iPP, Midilena III PPF401, with MFR = 3 g/10 min (230C, 2.16 kg) from Rompetrol Petrochemicals S.R.L. (Romania) was used. The selected iPP grade is characterized by a low modification level, i.e. there are no plasticizers, colored masterbatches or nucleating agents added, which is an ideal formulation to investigate the effect of nucleating agents and other additives on properties of iPP.

Two sorbitol derivatives were selected as additives for iPP: 1,3:2,4-bis(3,4- dimethylbenzylidene)sorbitol, Millad 3988 (DMDBS) and 1,2,3-tridesoxy-4,6:5,7-bis-O-[(4- propylphenyl)methylene]nonitol sorbitol, Millad NX8000 (NX8000), both were kindly provided by Milliken Chemical Company (Belgium, USA). Previous investigations revealed that 0.25 wt% is a sufficient amount of sorbitol able to promote



-crystal nucleation in isotactic polypropylene [41- 43]. Siloxane-silsesquioxane resin modified with phenyl groups of general formula [PhSiO1.5]n, abbreviated SiOPh, was synthesized in our laboratory as detailed below. The thermal stability of all materials was determined using thermogravimetry (TGA), which proved that all the components do not undergo thermal degradation at the used processing temperatures of iPP. The chemical formulas of DMDBS and NX8000 are presented in Figure 2.1.

(a) (b)

Fig. 2.1 Chemical structures of (a) DMDBS, (b) NX8000

2.2 Preparation of SiOPh

The synthesis was conducted using two-step acid-base co-condensation of

tetraethoxysilane and phenyltriethoxysilane mixture in 2:1 molar ratio. Tetraethoxysilane,

phenyltriethoxysilane and tetrahydrofuran (THF), in an amount of twice the volume of the silanes

used, were placed in a flask equipped with a mechanical stirrer. The solution was stirred for 30

min to homogenize the ingredients. Then, an aqueous solution of hydrochloric acid in water (1.5

mL of 35.5% HCl in 100 mL of water) was added into the solution of silanes in THF in order to

acidify the reaction medium. Once the acid was added, the reaction mixture became turbid. After

about 15 min, turbidity disappeared and a rise in temperature occurred. Stirring was maintained

for 60 min to cool down the mixture to room temperature. Next, aqueous ammonia solution was

added dropwise in order to basify the reaction medium (3.5 mL of 25% aqueous ammonia in 200

mL of water). Upon the addition of alkali, the mixture underwent turbidity again (formation of the

product). After the addition was completed, an intensive stirring was maintained for additional 60

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17

Experimental

min. Afterwards, the mixture was filtered, washed twice with 100 ml of water and dried for 24 hours at 120°C. The chemical formula of SiOPh is presented in Figure 2.2.

Fig. 2.2 Chemical structure of SiOPh

2.3 Sample preparation

Polypropylene pellets were milled into powder in a Tria high-speed grinder. Polymer was mixed with the sorbitol derivatives and SiOPh in the rotary mixer Retsch GM 200 for 3 min at a rotation speed of 3000 rpm. Homogenization of the premixed compositions with different SiOPh contents (0.1 – 3 wt%) and a fixed sorbitol concentration of 0.25 wt% was ensured by molten state extrusion with a Zamak corotating twin screw extruder operated at 190C and 70 rpm. The extruded rod was pelletized in a water bath. iPP/sorbitol/SiOPh composites at various compositions were prepared, as summarized in Table 2.1.

Table 2.1 Designations and mass concentrations of samples

Mass concentration [%]

iPP DMDBS SiOPh iPP NX8000 SiOPh

iPP 100 0 0 iPP 100 0 0

iPP+DMDBS 99.75 0.25 0 iPP+NX8000 99.75 0.25 0

iPP+DM+0.1SiOPh 99.65 0.25 0.1 iPP+NX+0.1SiOPh 99.65 0.25 0.1 iPP+DM+0.5SiOPh 99.25 0.25 0.5 iPP+NX+0.5SiOPh 99.25 0.25 0.5

iPP+DM +1SiOPh 98.75 0.25 1 iPP+NX+1SiOPh 98.75 0.25 1

iPP+DM +1.5SiOPh 98.25 0.25 1.5 iPP+NX+3SiOPh 96.75 0.25 3

The composites were compression-molded with a Collin Hydraulic Laboratory Forming

Press P 200 E at a temperature of 200°C, firstly without any pressure applied to allow complete

melting (3 min), followed by exerting a pressure of about 10 MPa for another 3 min, then cooled

to room temperature in less than 3 min by means of cold water circulating in the plates of the

press. iPP-based sheets with a thickness of 150 µm and 1.2 mm were obtained. The thin films

were used for the study of crystallization rate, to limit the effect of thermal gradients within the

sample during the phase transition, whereas the thick films were used for the investigation of

mechanical properties of the materials, to comply with ISO standards [44].

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18

Experimental

2.4 Methodology

2.4.1 Scanning Electron Microscopy (SEM)

Morphological analysis of cryogenically fractured iPP/sorbitol/SiOPh composites was performed using a FEI Quanta 200 FEG environmental scanning electron microscope (ESEM) (Eindhoven, The Netherlands) in low vacuum mode, using a Large Field Detector (LFD) and an accelerating voltage of 10-20 kV. Before analysis, the samples were sputtered-coated with Au-Pd alloy using Baltech Med 020 Sputter Coater System, then mounted on aluminum stubs.

2.4.2 Differential Scanning Calorimetry (DSC)

The thermal properties of iPP and its composites were investigated using a Mettler DSC 822 calorimeter, Mettler-Toledo, Inc., equipped with a liquid-nitrogen accessory for fast cooling.

The calorimeter was calibrated in temperature and energy using indium. Dry nitrogen was used as purge gas at a rate of 30 ml/min.

In order to set the structure for the analysis of crystallization kinetics, each sample was heated from 25 to 200°C at a rate of 20°C min

^-1

, melted at 200°C for 10 minutes to erase previous thermal history, quickly cooled to 160° at 20°C min

^-1

to limit as much as possible the exposure to high temperatures, then cooled at various rates, ranging from 0.5 to 4°Cmin

^-1

.

Crystallization is an exothermic process, and the heat evolved during the phase transition may cause thermal gradients within the sample. As a consequence, transitions can occur at temperatures that do not correspond to those detected by the instrumentation [45,46]. The thicker the sample, the more critical this problem is. In order to reduce these issues, sample mass was limited to 3.0 ± 0.2 mg, and cooling rates not exceeding 4°C min

^-1

were used (compare with 2.3).

2.4.3 Polarized Optical Microscopy (POM)

Spherulite growth rates were estimated by optical microscopy, using a Zeiss polarizing microscope equipped with a Linkam TMHS 600 hot stage. A small piece of compression-molded sample was squeezed between two microscope slides, then inserted in the hot stage. The thickness of the squeezed sample was lower than 10

m. The radius of the growing crystals was

monitored during solidification by taking photomicrographs at appropriate intervals of time, using a Scion Corporation CFW-1312C Digital Camera. Spherulite radii were measured with the software Image-Pro Plus 7.0.

The temperature program before isothermal and non-isothermal crystallizations were identical to those used in calorimetry. For non-isothermal crystallization, a self-nucleation procedure was used, in order to expand the temperature range of analysis. Each sample was heated from 25 to 200°C at a rate of 20°C min

^-1

, melted at 200°C for 10 minutes to erase previous thermal history, cooled to 155°C at 30°C min

^-1

, and maintained at 155°C until the appearance of the first crystals. Then, the temperature was raised to 160°C by heating at 30°C min

^-1

, equilibrated at 160°C, then the samples were cooled at 0.5°C min

^-1

until spherulite impingement [47].

Isothermal crystallization measurements at 150C and 153C were also conducted, to

confirm reliability of the data gained using the non-isothermal procedure.

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19

Experimental

2.4.4 Oscillatory Rheological Measurements

An MCR Anton Paar rheometer with a plate diameter of 25 mm operating in the plate- plate configurations under the oscillatory shear mode with a frequency  = 10 rad s

^-1

and a strain of 1%, was used in the rheological investigations. In each experiment the sample was melted at 220°C and held at for 10 min to erase memories of previous processing, as well as to reduce the internal stress associated to sample preparation. Afterward, the sample was cooled to the final temperature of 100°C at a constant cooling rate  = 4 °C min

^-1

. Crystallization was monitored using the oscillatory shear mode [48,49].

2.4.5 Fourier Transform Infrared Spectroscopy (FTIR)

Infrared spectra of pure SiOPh were collected with a Bruker Tensor 27 FTIR spectrometer in ATR mode using 16 scans. The spectra resolution was 2 cm

^-1

. Sample specimens were measured as obtained, without additional preparation steps.

The IR spectra were also collected with a Bruker Vertex 70 FTIR spectrometer in absorption mode using 64 scans. The spectra resolution was 1 cm

^-1

. Sample specimens were prepared by mixing SiOPh and DMDBS with milled potassium bromide (KBr). This powder was then compressed under vacuum into thin pellets using a pressure of 20 MPa. Four different mixtures were examined by FTIR. The first and second samples were prepared at ambient temperature using pure SiOPh and DMDBS, respectively. The other specimens were mixtures of SiOPh and DMDBS with a ratio of 1 : 1. For the tests, one sample was kept at ambient temperature and the second was heated to 190C. All of the spectra were submitted to a weather correction, including the correction for CO

2

[50-52].

2.4.6 Wide-Angle X-ray Diffraction (WAXD)

WAXD measurements were performed on a Philips Analytical X-Ray, model PW 1830 diffractometer with Cu Kα radiation. The scanned 2θ range was from 2 to 45 with a scanning rate of 0.02 and time per step of 1 s. The samples used for WAXS analysis were in the form of compression molded sheets of 1.2 mm thickness. Characteristic peaks were assigned according to the literature [7,16,53].

2.4.7 Haze

The optical behavior of the polymers was characterized by haze measurements carried out on the 150 µm compression molded films according to the PN-84/C-89100 standard [54] using a Haze Meter HM-150 produced by Murakami Color Research Laboratory (Japan).

2.4.8 Static Tensile Tests

Mechanical properties of iPP-based compression-molded sheets were evaluated as per

ISO 52732 using Instron tensile tester, model 4505. Measurements of elastic modulus were

conducted at a crosshead speed of 1 mm min

^-1

, to ensure deformation rate close to 1% of

measuring section length per minute [44]. Elongation at break was estimated at a speed of 50 mm

min

^-1

for a reasonable duration of the experiments [55,56].

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20

3. Results and discussion

3.1 Structure and crystallization behavior

Structure and morphology of the novel phenyl siloxane-silsesquioxane resin, synthesized in both University in Poznao and CNR laboratories, were investigated by electron microscopy and infrared spectroscopy. The SEM micrograph displayed in Fig. 3.1 shows spherical SiOPh particles with a diameter of approximately 1µm. It is also seen from the image that a sufficient size distribution of the particles was achieved using the two-step acid-base condensation method detailed above.

Fig. 3.1 SEM micrograph showing morphology of SiOPh particles

Figure 3.2 presents the FTIR spectrum of the SiOPh resin. All bands in the FTIR spectrum

confirm the structure of the obtained material. Broad bands at 3627 and 3390 cm

^-1

are attributed

to the presence of silanol groups (respectively free OH and hydrogen-bonded ones). The band at

954cm

^-1

characteristic for Si



OH groups is also observed in the spectrum. The bands at 1631 and

571cm

^-1

indices minor hydratation of the sample. The presence of phenyl groups is confirmed by

the presence of stretching vibrations of CH, C=C (3075, 3054, 1595 and 1431 cm

^-1

, respectively)

as well as bending vibrations of =CH bonds at 789, 738 and 697 cm

^-1

. A high intensity band, with

two maxima at 1130 and 1049 cm

^-1

, results from stretching vibrations of SiOSi bands and is

characteristic for silicon materials [57]. The maximum at 1049 cm

^-1

indicates the presence of

smaller stretching of SiOSi bands (<144) and the one at 1130 cm

^-1

confirms those of about

144. This split in oscillation region is related to the transverse and longitudinal valence

oscillations of the atoms in siloxane bridge so the frequencies of the mentioned oscillations are

distributed in separate regions from 1010 to 1140 cm

^-1

and 1190 to 1300 cm

^-1

in the range of θ

from 120 to 180° *58]. This split can also be attributed to the simultaneous presence of Q and T

units in the examined structure. The Q unit represents a silicon atom linked through siloxane

bond to subsequent four silicon atoms forming a tetrahedron. Wherein, the T unit presents a

silicon atom bonded to further three silicon atoms and one phenyl group [59]. The low intensity

bands characteristic for stretching vibrations of CH bonds (sp

³

CH

3

and sp

²

CH

2

groups) at about

2979 and 2929 cm

^-1

appeared due to the presence of small amount of unreacted (not hydrolyzed)

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21

3. Results and discussion

ethoxy groups. Obtaining such a structure, that is siloxane-silsesquioxane resin with POSS Q structures as network nodes with reactive phenyl groups in siloxane bridges between silsesquioxane (POSS) molecules, allows for replacement of expensive functionalized silsesquioxanes with this cheaper compound. Moreover, resin containing Q POSS units and reactive functional groups is, in principle, capable to interact with organic polymers and sorbitol derivatives, which has been studied in-depth and results are presented in this thesis.

Fig. 3.2 FTIR spectrum of SiOPh resin

To gain information about the phase structure and morphology of the iPP composites containing sorbitols and resin, scanning electron microscopy analyses were performed. The results are presented in Figure 3.3 and 3.4 Figures 3.3-a and 3.4-a illustrate the cryogenically- fractured surface of compression-molded plain iPP, which appears quite smooth as expected.

Morphology of iPP composite containing 0.25 wt% of NX8000 is exhibited in Figure 3.3-b. The

fractured surface is quite smooth and similar to the one shown in Figure 3.3-a, with only small

particles that adhere on iPP surface, possibly revealing the presence of sorbitol. The latter

appears well distributed within iPP, thorough the analyzed area. The fractured surface

morphology changes drastically upon addition of SiOPh, as illustrated in Figure 3.3-c, which

presents the scanning electron micrograph of the sample containing 3 wt% of resin. A number of

large voids of various dimensions are visible on the sample surface, possibly due to filler particles

that are pulled out upon the cryogenically fracture process. At the same time some nearly

spherical particles, attached to the iPP matrix also appear. The size of both the holes and particles

indicates considerable filler agglomeration upon melt processing, since the average particle size of

unprocessed SiOPh is ~1µm, as shown in Figure 3.1, much smaller that the spheres and voids seen

in Figure 3.3-c.

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22

3. Results and discussion

Fig. 3.3 SEM micrographs for iPP (a), iPP+NX8000 (b), iPP+NX+3SiOPh (c)

Polypropylene nucleated with DMDBS reveals denser network of lines visible on sample surface (Fig. 3.4-b). Morphology seems to be homogeneous as the nucleating agent well dissolves in iPP while processing [28]. However, small spherulite size and minute amount of DMDBS added do not allow to distinguish particular components in the fractured sample presented in SEM micrograph. Figure 3.4-c illustrates morphology of the composite containing both modifiers, which significantly differs from the fractured surfaces shown in Figures 3.4 and 3,4-b. Their average diameter is about 1 m, which well compares with the average size of the as-synthesized SiOPh. This indicates that negligible agglomeration of the fillers occurred upon melt processing of iPP/DMDBS/SiOPh composites. The modifier is nicely distributed and attached to the studied surface, which reveals good distribution of the filler within the sample.

Fig. 3.4 SEM micrographs for iPP (a), iPP+DMDBS (b), iPP+DM+1.5SiOPh (c)

Calorimetric studies were realized by differential scanning calorimetry measurements at four different cooling rates, to obtain in-depth information on the crystallization behavior of iPP composites. The raw DSC curves for the cooling rate of 4C/min are presented in Fig. 3.5-a and 3.6-a. Plain iPP starts its crystallization at 129C, when cooled at 4C/min. Upon the addition of DMDBS and NX8000 sorbitol derivatives the onset of crystallization is shifted towards higher temperatures, i.e. to 138



C and 137



C, respectively. This proves that both sorbitol derivatives enhance nucleation of iPP spherulites, which occurs at higher temperatures, as expected [19,28,42].

Addition of the siloxane-silsesquioxane resin to iPP/DMDBS formulation slightly reduces

the crystallization temperature of iPP, compared to the one containing iPP and DMDBS only. A

significant decrease of the crystallization temperature range occurs in the presence of 1.5 wt% of

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23

3. Results and discussion

SiOPh, as seen in Figure 3.5-a. The influence of SiOPh/DMDBS composition on nucleation efficiency of DMDBS is quantified in Fig. 3.5-b, which presents the DSC onset temperature of crystallization as a function of the cooling rate. The onset point was taken as the intersection of the baseline before the transition and the inflectional tangent and the peak temperature as the maximum of the exotherm [60]. As seen in Figure 3.5-b, the onset of the phase transition shifts towards higher temperatures with decreasing cooling rate , as typical for polymer crystallization:

lower

 ensures more time to overcome the energy barriers for nucleation [45]. More

importantly, at parity of cooling rate, compared to iPP containing DMDBS only, increasing amounts of SiOPh induce crystallization to start at progressively lower temperatures, indicating that the presence of the siloxane-silsesquioxane resin significantly affects nucleating efficiency of the sorbitol derivative, with a content of 1.5 wt% of SiOPh being the most effective one.

(a)

(b)

Fig. 3.5 DSC crystallization curves and the onset temperature versus cooling rates

The addition of 0.25 wt% of NX8000 sorbitol induces crystallization to start at high

temperatures compared to plain iPP. Further incorporation of 0.1 to 3 wt% of SiOPh into

iPP/NX8000 does not induce significant variations in the crystallization profile of iPP nucleated

with sorbitol, as all the DSC curves shown in Figure 3.6-a practically overlap, within experimental

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24

3. Results and discussion

error. The onset temperatures of the analyzed composites are plotted in Figure 3.6-b as function of the cooling rate. In polymer samples containing added particles, for a given cooling rate, the temperature at which crystallization starts is indicative of the effectiveness of the filler to promote heterogeneous nucleation [61,62].

(a)

(b)

Fig. 3.6 DSC crystallization curves and the onset crystallization temperature versus cooling rates

Sorbitol clarifiers dissolve in molten iPP and thereby, distributes homogenously during

processing through the whole volume of polymeric matrix. As proposed in Ref. [41,63,64],

intermolecular hydrogen bonding and π_π interactions between the adjacent phenyl rings are

established upon cooling, and sorbitol crystallizes with development of a percolated fibrillar

network that facilitates growth of polypropylene spherulites [28]. As mentioned above, the

increased crystallization temperature of the iPP/sorbitol composites proves that 0.25 wt% is a

sufficient amount to allow precipitation of sorbitol upon cooling. Progressive addition of SiOPh

into iPP/DMDBS formulation causes a corresponding delay in crystallization, quantified in Figure

3.5. Instead, when the siloxane-silsesquioxane resin is added to the iPP/NX8000 formulation, the

nucleation efficiency is not affected. Both sorbitols and SiOPh, which structures are depicted in

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25

3. Results and discussion

Figures 2.1 and 2.2, have functional groups that may in principle interact upon melt mixing, by possible establishment of hydrogen bonds, influencing miscibility of the modified sorbitol with polypropylene and in turn, its efficiency as nucleating agent [30]. It was proved [35] that the fibrils of sorbitol derivatives have higher affinity to molecules modified with silanol groups. However, for all the analyzed compositions containing NX8000 and various amounts of SiOPh, the onset temperature is not affected by SiOPh, as shown in Figure 3.6-b. This indicates that limited or negligible interaction between NX8000 and SiOPh is established, and the modified sorbitol maintains its efficiency as nucleating agent for isotactic polypropylene, independently of resin content, at least for the analyzed composition range. On the other hand, some kind of interplay between DMDBS and SiOPh is established, however the modified sorbitol maintains its efficiency as nucleating agent for isotactic polypropylene, at least for the analyzed composition range, i.e.

below 1.5 wt% of SiOPh.

Tables 3.1 and 3.2 summarize characteristic values obtained during non-isothermal crystallization of iPP composites. The onset temperatures are pictured in Figures 3.5-b and 3.6-b, hence are not repeated in Tables 3.1 and 3.2, which only show the peak temperatures (T

p

) and crystallinity degrees (X

c

).

Table 3.1 Crystallization temperature peaks (T

p

) of iPP-based formulations cooled at the indicated rates

Cooling rate [C/min] Cooling rate [C/min]

4 2 1 0.5 4 2 1 0.5

iPP 127.0 129.5 132.5 135.5 _iPP 127.0 129.5 132.5 135.5

iPP+DMDBS 134.5 137.0 139.0 141.0 iPP+NX8000 134.5 136.5 138.0 139.5 iPP+DM+0.1SiOPh 134.0 136.5 139.0 141.0 iPP+NX+0.1SiOPh 134.0 136.5 138.5 140.5 iPP+DM+0.5SiOPh 133.5 135.5 138.0 141.0 iPP+NX+0.5SiOPh 134.5 136.5 138.0 139.5 iPP+DM+1SiOPh 133.5 135.5 136.0 139.0 iPP+NX+1SiOPh 134.0 136.0 138.0 140.0 iPP+DM+1.5SiOPh 127.5 131.5 134.0 136.0 iPP+NX+3SiOPh 134.5 136.0 138.0 139.5

The crystallinity degree values were calculated from the data obtained from the DSC plots, gained during cooling at different rates, according to the following equation (3.1):

-

(3.1)

where  H

M

is the experimental heat of fusion,  H

0

is the enthalpy of fusion of the fully crystalline

polymer, equal to 207.1 Jg

^-1

[65], and  is the weight fraction of the modifiers. As reported in

Table 3.2, the crystal fraction developed upon cooling at rates up to 4°Cmin

^-1

is X

c

= 50

 3%,

independently of composite composition. This indicates that the addition of sorbitol only, or

coupled with up to 3 wt% of SiOPh, does not affect crystallinity of polypropylene, under the

chosen experimental conditions. It influences only crystallization kinetics of iPP, as the onset and

peak points are shifted to higher temperatures due to enhanced nucleation of iPP spherulites.

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26

3. Results and discussion

Table 3.2 Crystallinity (X

c

) of iPP-based formulations cooled at the indicated rates

Cooling rate [C/min] Cooling rate [C/min]

4 2 1 0.5 4 2 1 0.5

iPP 51 48 48 53 _iPP

51 49 48 54

iPP+DMDBS 50 50 50 51 iPP+NX8000

50 52 50 53

iPP+DM+0.1SiOPh 52 49 49 51 iPP+NX+0.1SiOPh

51 51 51 53

iPP+DM+0.5SiOPh 52 47 47 47 iPP+NX+0.5SiOPh

51 50 48 52

iPP+DM+1SiOPh 52 49 49 51 iPP+NX+1SiOPh

50 49 51 51

iPP+DM+1.5SiOPh 47 50 50 48 iPP+NX+3SiOPh

51 51 53 52

Polarized Optical Microscopy (POM) was used to estimate the spherulite growth rate (G) of iPP and its composites with sorbitols and resin. In order to gain data in a wide temperature range, non-isothermal crystallization experiments were performed using a self-nucleation procedure [46]. Such method was chosen because of the high density of nucleation of the samples that contain sorbitol: at low temperatures high number of very small spherulites grow simultaneously, which hinders reliable analysis of the rate of their growth. Conversely, with self- nucleation and measurements upon continuous cooling, experiments are made when only a few iPP spherulites grow; they can reach relatively large dimensions, which permits reliable data analysis until impingement.

The measurements were conducted at a constant cooling rate so G can be estimated by taking the first derivative of the radius (r) vs. temperature (T) plot at each experimental point: