Uncovering the Unexpected Role Chain Connectivity has on Altering Dynamics of Thin Glassy Polymer Films Near Interfaces Open Access
Thees, Michael (Spring 2020)
Abstract
Polymer glasses are a widely studied system in thin films, with perturbations to dynamics due to the presence of interfaces being a topic of ever growing interest. Such interface confinement effects have generally been understood as being enthalpic in nature, with these property changes at interfaces forming a gradient in dynamics propagating into the film. However, recent studies have begun reporting differences in confinement effects associated with chain length, stiffness, and architecture indicating that entropic effects may play a modifying role to dynamics near interfaces. Here, we explore the effects that chain connectivity and entropy have on the local and film average glass transition dynamics near interfaces.
We measured the average glass transition temperature (Tg) and aging rate (β) in bulk (>200 nm thick) and thin 31 nm thick films for molecular weights (MWs) up to 10,100 kg/mol, where β is observed to decrease in all thin films. For thin films of ultra-high molecular weight PS (MWs>6,500 kg/mol) an increased β is observed compared to thin films of merely high molecular weight PS (MWs<3,500 kg/mol). This increase in β for thin films is attributed to a diminished gradient in dynamics at ultra-high molecular weights, associated with chain connectivity.
Motivated by studies recently proposing that chain adsorption may be altering dynamics in thin films, we evaluate the procedure for forming adsorbed layers by washing off unadsorbed chains from melt annealed films, called the “Guiselin experiment”. We demonstrate that the adsorbed amount measured is highly dependent on the substrate cleaning and solvent washing protocol used. Measuring the effect that these adsorbed layers have on average dynamics, we observe no significant change to either the average Tg or average β. Adsorption and desorption of layers formed directly in solutions was also investigated at different concentrations for comparison with those formed by washing off melt annealed films. Local Tg adjacent to adsorbed layers formed by washing off melt films for different times or grown directly in solution were examined to determine how different adsorbed layers affect local Tg. We found that the local Tg adjacent to adsorbed layers only increased for adsorbed layers formed by washing for short times well-annealed melt films, provided good chain interpenetration was obtained for the system. Adsorbed layers well equilibrated in solution showed no corresponding changes in Tg.
Table of Contents
Contents
1 Introduction and Scope 1
1.1 Introduction to Polymer Glasses . . . . . . . . . . . . . . . . . . . . . 1
1.2 Confinement Effects in Thin Polymer Films . . . . . . . . . . . . . . 7
1.3 Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2 Unexpected Molecular Weight Dependence to the Physical Aging of
Thin Polystyrene Films Present at Ultra High Molecular Weights 13
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.2 Experimental Methods . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.3.1 Qualitative observations of differences in physical aging for thin
films of ultra-high molecular weight polystyrene . . . . . . . . 19
2.3.2 Quantitative description of differences in physical aging for thin
films of ultra-high molecular weight polystyrene . . . . . . . . 24
2.3.3 Attempting to understanding differences in physical aging for
thin films of ultra-high molecular weight polystyrene . . . . . 33
2.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
3 Review and Reproducibility of Forming Adsorbed Layers From Solvent
Washing of Melt Annealed Films 46
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
3.1.1 Summary of Recent Investigations on Adsorbed Layers Formed
from Melt Films . . . . . . . . . . . . . . . . . . . . . . . . . . 48
3.1.2 Summary of the Understanding Developed of Chain
3.2 Experimental Methods . . . . . . . . . . . . . . . . . . . . . . . . . . 63
Adsorption in Solution . . . . . . . . . . . . . . . . . . . . . . 55
3.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 66
3.3.1 Reliability of Measuring Adsorbed Layer Thickness hads with
Ellipsometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
3.3.2 Reproducibility of Measuring Adsorbed Layer Thickness hads . 70
3.3.3 Impact of Multiple ofWashings and Substrate Cleaning Method
on Adsorbed Layer Thickness hads . . . . . . . . . . . . . . . . 80
3.3.4 Impact of Adsorbed Layers on Film Average Tg(h) and Physical
Aging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
3.4 Conclusions: Summary, Implications and Open Questions . . . . . . . 94
3.4.1 Summary of Findings . . . . . . . . . . . . . . . . . . . . . . . 95
3.4.2 Open Questions . . . . . . . . . . . . . . . . . . . . . . . . . . 99
4 Role of Solvent Washing Conditions on the Creation of Adsorbed
Layers from Melt Films and Its Impact on Local Tg Dynamics 104
4.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
4.2 Experimental Methods . . . . . . . . . . . . . . . . . . . . . . . . . . 107
4.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 109
4.3.1 Formation and characterization of adsorbed layers by different
methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
4.3.2 Formation and characterization of adsorbed layers by different
methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
4.3.3 Local Tg(z = 0) next to an adsorbed layer . . . . . . . . . . . 119
4.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
5 Outlook and open questions 132
Bibliography 136
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