Major Impact of Initiator Fragments Located at Chain Ends on the Glass Transition Temperature and Fragility of Low Molecular Weight Polystyrene

Major impact of initiator fragments located at chain ends on

the glass transition temperature and fragility of low molecular weight polystyrene

Lanhe Zhang¹ and John M. Torkelson^1,2

¹Dept. of Materials Science and Eng., Northwestern University, Evanston, IL 60208, USA

²Dept. of Chemical and Biological Eng., Northwestern University, Evanston, IL 60208, USA

Contact information

Lanhe Zhang – lanhezhang2012@u.northwestern.edu

John M. Torkelson – j-torkelson@northwestern.edu

Abstract

Important, but unexplored effects of chemically distinct initiator fragments incorporated as chain ends during free radical polymerization (FRP) of styrene on material properties are investigated in depth. While the effect of different chain ends on T_g is negligible for high molecular weight (MW) PS, it is substantial in ultralow MW PS due to the molar enrichment of chain ends. Such an investigation is critical in light of the rising technological and scientific interests in developing novel photoresist materials and functional coatings that are based on relatively low MW polymers. Particularly, the need for high resolution and low line-edge roughness of features formed during microlithographic processing has led to the use of polymers and copolymers with MW typically well below 10 kg/mol. In addition, low MW functionalized polymers exhibit higher grafting density/better adsorption to solid/liquid interfaces due to higher interfacial diffusion rate and interfacial reaction rate.

Changes in material properties are expected with decreasing MW of a polymeric system as a consequence of increased chain-end concentration. However, almost no studies have investigated the influence of initiator fragments as chain ends on the bulk material properties of polymers synthesized by FRP. We study for the first time the effect of chemically distinct initiator fragments incorporated as chain ends during RP on T_g, fragility, and surface wettability of ultralow MW PS. Polystyrene samples with a wide range of MW down to ~4,000 g/mol were synthesized via free radical and pseudo-living polymerization. Initiators ranging from highly polar and bulky 4,4'-azobis(4-cyanovaleric acid) (ACVA) to highly flexible, nonpolar n-dodecyl mercaptan (RSH) were employed to obtain a variety of chain-end structures. NMR and FTIR spectroscopy confirmed the molar enrichment of initiator fragments at chain ends in ultralow MW PS samples. For example, NMR analysis determined that nearly all chain ends in PS/BPO/3.5k were initiator fragments, consistent with the fact that PS undergoes termination predominantly via combination.

The influence that such chemically distinct chain ends have on the MW dependence of glass transition temperature and fragility, commonly understood to be a result of excess free volume/configurational entropy conferred by chain ends, has not been explored beyond speculation. For ultralow MW PS of M_n ~4,000 g/mol, the chain-end structures can tune the T_g,onset values over a 33 °C range going from 61 °C for PS/RSH/3.7k to 94 °C for PS/ACVA/4.1k. The presence of long, flexible dodecane ends at substantial levels can explain the dramatically lowered T_g for PS/RSH/3.7k, while bulky chain ends with hydrogen bonding capabilities cause the near MW independence of T_g as exhibited in PS/ACVA. Fragility (m) was also measured on samples with M_n ~4,000 g/mol to understand the impact of such chain ends on the cooperativity and chain packing effectiveness. The presence of non-polar, flexible dodecane ends in PS/RSH/3.7k makes it a stronger glass former, experiencing much less packing frustration (m = 65) compared to PS/Anionic/3.6k (m = 90). On the other hand, incorporation of other bulky end groups such as cyano isopropyl and benzoyloxy resulted in moderate to no increase in fragility, respectively compared to the PS standard. This can be explained based on the minor contribution offered by these chain-end structures to the overall steric hindrance of the rigid styrene repeat unit. Interestingly, swapping the isopropyl groups in 2,2'-azobis(2-methylpropionitrile) with cyclohexane ring in 1,1'-azobis(cyclohexanecarbonitrile) resulted in a larger m compared to PS standard, possibly due to the larger van der Waals volume possessed by the ring structure. The largest m of 130 was measured in PS/ACVA/4.1k due to the ability for the -COOH groups in the cyano pentanoic acid ends to hydrogen bond. The additional interchain forces arising from such interactions make the material a more fragile glass former with fragility that is twice that of PS/RSH/3.7k and similar to 20,000 g/mol PS standard reported in literature.    

These results further shine light on the distinction between entanglement MW and the MW below which bulk T_g becomes MW dependent. Researchers have often attempted to attribute the molecular weight dependence of T_g in polymers to chain entanglement. However, the current study definitively demonstrates that the MW dependence of T_g in PS has no relation with chain entanglement. Regardless of chain-end structure, the MW between entanglements (M_e) for PS is 18,000 g/mol. At this MW, PS/ACVA has a T_g of ~102 °C and maintains the same T_g (within experimental error) until M_n ~5,200 g/mol. On the other hand, PS/RSH with M_n ~18,000 g/mol has a T_g of ~90 °C, almost 10 °C lower than that seen with its high MW counterpart.

Lastly, these ultralow MW materials are examined on their surface wettability, a parameter sensitive to changes in surface chemical composition. By incorporating polar initiator fragments at chain ends, films of ultralow MW PS exhibit tunable surface wettability. Due to an enrichment of -COOH chain ends, θ_R, an indicator for surface hydrophilicity, is decreased by almost 30° for PS/ACVA/4.1k compared to its high MW counterpart. This makes evident of the strong retention of water on the polymer surface due to the presence of substantial levels of          -COOH groups at chain ends that undergo hydrogen bonding with water.