AmericanChemicalSociety.com

This report summarizes our activities related to the project funded by the PRF-G program, in which we are studying block copolymer/layered silicate nanocomposites (BCPLSs), where the block copolymers are grafted to the surface of fully exfoliated montmorillonite (MMT) clay particles.

Activity I: Kinetics of graft-from atom transfer radical polymerization (ATRP).

Our original hypothesis stated that the graft-from (surface initiated) ATRP should provide a convenient route to fully exfoliated clay/polymer nanocomposites. Our initial experiments indicated that this is only true in situations where the reaction is not mass transport limited, and accordingly precise control over the polymerization kinetics are critical. Our experiments with polystyrene (PS) and poly(t-butyl acylate) (PtBA) demonstrated unequivocally that the polymerization rate of a surface-initiated ATRP is nearly an order of magnitude greater than that of an analogous bulk polymerization.

To understand this auto-acceleration effect we prepared a number of MMT specimen with a controlled areal density of ATRP-active sites and carefully measured the polymerization kinetics as a function of graft density and temperature. Our results indicated that the controlling parameter is the graft density, and bulk kinetics are rapidly recovered as the graft density falls below about .25 chains per square nanometer. We rationalized these data on a basis of local catalyst concentration fluctuations that effectively shift the ATRP equilibrium to a higher radical concentration, and developed a phenomenological model with a single fitting parameter that quantitatively accounted for our data. These findings allowed us to appropriately tune our reaction conditions such that we were able to obtain fully exfoliated MMT/polymer brushes as per our hypothesis.

These results were published in Macromolecules.

Activity II: Thermodynamic behavior of BCPLS materials.

A series of MMT-PS homopolymer brushes varying PS molecular weight at maximal graft density were prepared, and subsequently repolymerized to add a PtBA block, yielding MMT-PS-PtBA block copolymer brushes over a range of composition and total molecular weight. Further tuning of catalyst concentration and halogen species was required to yield specimen with polydispersity < 1.2.

Currently, we have fully characterized a subset of these materials, paying particular attention to the conditions under which microphase separation is present and understanding how BCPLS materials differ from their traditional diblock copolymer analogues.

A number of striking phenomena have been discovered. First of all, microphase separation occurs in BCPLSs only at significantly stronger degrees of segregation strength than in the corresponding block copolymers. This may be understood in view of the fact that by virtue of their dense grafting (1 chain per square nanometer), the chains are very strongly stretched and thus the chain stretching contribution to the BCPLS free energy is amplified compared to diblock copoymers. Secondly, when microphase separation does occur, the associated domain sizes are on the order of 5 times that of the analogous diblock copolymer. Again, this may be understood in terms of large extent to which chains are extended; in most cases domain sizes are on the order of 25% of the contour length, or 200-300nm. This finding is particularly interesting since this materials platform evidently offers a route to materials that could interact with visible light at molar mass of < 200 kDa, compared to traditional diblock copolymers where > 1 MDa is required to reach this length scale.  Finally, the molecular weight plays a significant role in the nature of the morphology, in contrast to traditional block copolymers where the composition is the only major factor. At lower molar mass, chains on discrete clay particles are unable to fully “communicate” with neighboring particles and the resultant structure indicates self-assembly in the plane defined by the attached particles, with little influence on the ordering of the particles themselves. We have termed this mode of self assembly “interparticle” microphase separation. When the total molar mass reaches a critical threshold, which we hypothesize is directly related to the mean particle size, the character of the morphology changes drastically and both local polymer self-assembly and long-range particle assembly is evident; this regime we refer to as “interparticle” self assembly. We have prepared a manuscript that communicates our preliminary findings on this topic, currently under consideration for publication in Advanced Materials; Macromolecules is a secondary target in the event that our manuscript fails to survive the review process of this highly prestigious journal.

Activity III: Thermal effects in BCPLSs

DSC traces in our numerous BCPLS specimen indicate a remarkable finding: the glass transition temperature of PtBA is either very near its bulk value of 43 degrees Celsius, or at temperatures as high as 69 degrees. This elevation is strongly correlated to the morphology, with the higher value in systems exhibiting “interparticle” self-assembly. We are in the process of devising other materials systems to further expose this thermal effect. Poly(n-butyl acrylate) has been identified as an alternative to PtBA since its bulk glass transition is well below room temperature. Block sequence is also an important consideration. Once we have validated the universality of this effect we will conduct a series of targeted experiments to elucidate the precise mechanism.

Future Work

Our understanding of BCPLS thermodynamics is still preliminary. We have a number of outstanding specimens to examine, and it will be an important objective to demonstrate the universality of the findings we have observed to date. The role of graft density on the morphology is an additional variable that we have not yet explored. As our understanding of formulation-morphology relationships continue to mature, we will begin to undertake the task of elucidating morphology-property relationships, particularly mechanical properties.