Partial Crystallinity in Alkyl Side Chain Polymers Dictates Surface Freezing

We have studied the structure of a novel crystalline surface monolayer on top of a disordered melt of the same material [poly($n$-alkyl acrylate)s] using grazing incidence x-ray diffraction. The grazing incidence x-ray diffraction, surface tension, and bulk latent heat results show that side chains crystallize except the nine methylene units of the alkyl side chains closest to the polymer backbone. The partial crystallinity along with a thicker surface layer, due to the additional length of the linker group, explains why the difference between the surface order-to-disorder transition temperature and bulk melting temperature increases with a decrease in the length of the alkyl side chain.

Figure 1

(a) The chemical structure of poly($n$-alkyl acrylate), where $n=16$, 18, and 22. (b) The schematic geometry of the x-ray experimental setup. The angle of incidence is ${\alpha }_i$, the reflection angle is ${\alpha }_r$, and the scattering angle is ${\alpha }_f$.

Figure 2

(a) Image of diffraction pattern on a plate detector for poly($n$-alkyl acrylate) in the surface crystalline phase ($T_m<T<T_{s1}$). The x-ray intensities along the Bragg rods for $n=16$ (b), 18 (c), and 22 (d) are plotted as a function of ${\alpha }_f/{\alpha }_c$. Shown as solid lines are fits to the data using Eq. (1). The dashed lines indicate calculated intensity profiles for $D=\mathrm{\text{extended}}$ side chain length. The inset in (b) compares the intensity dropoff along the arc (solid line) and along constant $q_r$ (dashed line) for a Gaussian distribution with $\sigma =8^{°}$ tilt, $D=8\AA$, and correlation length of 100 Å.

Figure 3

The experimental values of $\Delta T$ for poly($n$-alkyl acrylate)s (○) [11, 14] as a function of $n$. The predictions of $\Delta T$ using Eq. (2) are plotted after taking into consideration only the contribution of the extra length of the linker group in the surface ordered phase (dotted line) or the partial crystallinity of the surface side chains (dashed-dotted line). The predictions after taking into account both of these contributions are plotted as a solid line. We have also plotted the data for $n$-alkane (▵) and predictions (dashed line) for comparison [using ${\gamma }_{T_{s,\mathrm{alk}}}=28\mathrm{mN}{\mathrm{m}}^{-1}$, ${\gamma }_{T_m}=(22+1200/{\mathrm{n}}^2)\mathrm{mN}{\mathrm{m}}^{-1}$, and $(d\gamma /dT{)}_{T<T_{s,\mathrm{alk}}}=[k_B\ln (3^{n-3})\times {10}^3/(19.7\times {10}^{-20})-0.09]\mathrm{mN}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$] [9].