Molecular Weight Dependence of the Intrinsic Size Effect on $T_{\mathrm{g}}$ in AAO Template-Supported Polymer Nanorods: A DSC Study

We have investigated how the intrinsic size effect modifies the glass transition temperature ($T_{\mathrm{g}})$ and fragility of template-supported polystyrene (PS) nanorods in the absence of free surfaces and attractive polymer-substrate interactions. Template-supported nanorods of different molecular weight (MW) were prepared by melt infiltration; rod diameter ($d)$ varied from 24 nm to 210 nm. The $T_{\mathrm{g}}$- and fragility-confinement effects$_{\mathrm{\thinspace }}$were characterized using differential scanning calorimetry. $T_{\mathrm{g}}$-confinement effects are observed for PS nanorods MW $\ge $ 900 kg/mol; greater perturbations to~$T_{\mathrm{g}}$~are observed with increasing MW ($e.g.$, in 24 nm rods,~$T_{\mathrm{g,rod}}$~--~$T_{\mathrm{g,bulk}}$~$=$ -3.2 $^{\circ}$ C for 900 kg/mol PS, whereas~$T_{\mathrm{g,rod}}$~--~$T_{\mathrm{g,bulk}}$~$=$ -7.4 $^{\circ}$ C for 3,840 kg/mol PS). Intrinsic size effects can account for the MW-dependent behavior in PS nanorods. Comparing the length scale of the confining dimension, $d$, to that of the polymer radius of gyration,~$R_{\mathrm{g}}$. Perturbations to~$T_{\mathrm{g}}$~were observed when $d$ \textless 2$R_{\mathrm{g}}$. This result indicates that changes in polymer chain conformations due to confinement is important in perturbing~$T_{\mathrm{g}}$~in template-supported PS nanorods. We also determined that the $T_{\mathrm{g}}$-confinement is accompanied by a fragility-confinement effect. The fragility of 33-nm-diameter, 2,000 kg/mol PS nanorods is reduced 24{\%} from the bulk value. \quad