Acrylic pressure-sensitive adhesives with various polyester side-chain lengths were synthesized to investigate the effect of branching on phase separation and polymer mechanical performance. The polyester macromonomers (MMs) were produced through ring-opening co-polymerizations of l-lactide (l-LA) and ϵ-caprolactone (ϵ-CL) initiated with 2-hydroxyethyl methacrylate (HEMA), which provides the polyester chains with terminal vinyl groups. By varying the HEMA content, a range of MM chain lengths constructed from L10C4 (five l-LA and four ϵ-CL units) to L100C40 were obtained at a constant monomer mole ratio. Copolymerization of 2-ethylhexyl acrylate and acrylic acid with these MMs at constant mass composition provided a series of comb copolymers consisting of acrylic backbones with polyester branches of various chain lengths. Characterization of thin films cast from the polymers using thermal analysis and scanning probe microscopy showed a transition from a homogeneous phase to the formation of distinct microphases with increasing branching chain lengths. Rheological analysis of the linear viscoelastic responses was also used through small-amplitude oscillatory shear, and dynamic master curves were constructed by time-temperature superposition. The rheological data were also consistent with phase separation for the longer side-chain lengths of L50C20 and L100C40. The extra elastic contribution at low frequency and the temperature dependence of aT both show obviously effect of separated phases. Performance testing of polymer films showed that the chain extension resulted in a significant increase in both peel strength and shear resistance, which was accompanied by a modest decrease in film tackiness. The results demonstrate that tailoring branch chain structures provide a promising means for controlling the properties of the high-biomass content adhesive polymers.
Bibliographical noteFunding Information:
The authors acknowledge National Key Research and Development Program of China (2016YFB0302400), National Natural Science Foundation of China (21533611, 21420102008, and U1462115), and Chinese State Key Laboratory of Chemical Engineering at Zhejiang University (SKL-ChE-15D03) for financial support.
Copyright © 2018 American Chemical Society.