Limited Fraction of Crystallized Side Chains in Bottlebrush Poly(n-alkyl methacrylate)s
Crystallinity of bottlebrush polymers due to side chain crystallization has been considered to be related to the length of the side chains only under the assumption of complete participation of crystallization by all side chains. Recent experimental results revealed that in poly( n -alkyl methacrylate)s a fraction of side chains could not crystallize due to constraints imposed by the trapped main chain entanglements and required expansion of main chain-main chain distance. This result renders the original simplified consideration of the origin of crystallinity in bottlebrush polymers questionable. In this work, we introduce a new parameter f c , the fraction of crystallizable side chains, to better describe the crystallinity of bottlebrush polymers. A linear relationship between the melting enthalpy and the number of alkyl groups in side chains for bottlebrush polymers reported repeatedly indicates that f c remains essentially unchanged when bottlebrush polymers had the same main chain structure and grafting degree but different side chain lengths. The slope of the above-mentioned linear relationship is thus \(\Delta H_{\text{CH}_2}\times f_{c}\) , where \(\Delta H_{\text{CH}_2}\) stands for the melting enthalpy of one mole alkyl group packed into the crystal. With a known value of f c , it is possible to estimate the value of \(\Delta H_{\text{CH}_2}\) . In case of poly( n -alkyl methacrylate)s, we estimated \(\Delta H_{\text{CH}_2}\) of hexagonal crystal being at most 5.74 kJ/mol with the knowledge of possibly smallest f c of 0.67 obtained from small angle X-ray scattering data. Therefore, the crystallinity of bottlebrush polymer would be calculated based on the equation \(X_{\rm{c}}=f_{\rm{c}}\times\frac{N_{\rm{c}}}{N}\) with N and N c being the number of alkyl groups in a side chain and those packed in the crystalline structure, respectively. Both chemical structure and grafting degree of bottlebrush polymers affect f c .
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