Supercritical carbon dioxide (scCO(2)) has been used as the processing medium to facilitate effective impregnation and subsequent polymerization of methyl methacrylate monomer in ultrahigh molecular weight polyethylene (UHMWPE), providing a route to new nanocomposite materials. In this paper, the morphological structures of UHMWPE, scCO(2)-treated UHMWPE, and A blend of (53/47 w/w) UHMWPE/poly(methyl methacrylate) (PMMA) prepared in scCO(2) are investigated using tapping mode atomic force microscopy (TMAFM). Under moderate tapping conditions, phase imaging offers a good contrast of the nanostructures of the semicrystalline polymers due to a large difference in stiffness between the crystalline and amorphous phases. This is demonstrated by a TMAFM force mode characterization-method through amplitude-phase-distance (APD) measurements on the nanoscale crystalline and amorphous domains. The use of this method has also identified that the UHMWPE/PMMA blend consists of three phases: UHMWPE crystalline, UHMWPE amorphous, and PMMA amorphous. These three phases display three different characteristic tip-sample interaction profiles. The phase-segregated PMMA domains are estimated to be between 10 and 100 nm in size. In addition, it is found that the blending method via in-situ polymerization has significantly increased the crystallinity of the UHMWPE. The increase in crystallinity, together with the appearance of a second low melting temperature endothermic peak in differential scanning calorimetry (DSC) analysis of the UHMWPE/PMMA blend, is consistent with the increased population of small crystallites observed in the AFM phase images. We attribute this effect to the enhanced mobility of UHMWPE molecules caused by scCO(2) impregnation and addition of the PMMA component. Through statistical. analysis of the phase distribution in the APD data our results also demonstrate that the crystallinity of polymers can be quantified by using phase imaging analysis at 48.3% for UHMWPE, 52.2% for treated scCO(2), and 32.8% for UHMWPE/PMMA (i.e., 61.9% of the UHMWPE component). These data are within 5% agreement with standard DSC bulk analysis of crystallinity.