The concept of morphotropic phase boundary has recently been employed to enhance the piezoelectric response in polymeric ferroelectrics, substantially improving the $d_{33}$ coefficient through engineered phase boundaries between $\beta$ and $3/1$-helical phases in polyvinylidene fluoride-based systems. However, this strategy has so far been effective primarily for the converse piezoelectric effect, with negligible impact on the direct piezoelectric response. This is a critical limitation as most practical applications of ferroelectric polymers rely on direct piezoelectricity (e.g., sensing, energy harvesting). Here, we reveal that during dipole reorientation from the $3/1$-helical to the $\beta$ conformation, the pronounced disparity between weak interchain van der Waals cohesion and strong intrachain covalent rigidity triggers an unexpected lateral expansion of the interchain spacing. This intermediate expansion raises the energy barrier for dipole reorientation under compressive stress, the very condition central to direct piezoelectric operation. To overcome this barrier, we apply a bias electric field combined with constant tensile stress along the $c$ axis, deliberately inducing a moderate interchain lattice expansion in advance. This preexpansion effectively lowers the energy barrier for subsequent dipole rotation under compression, thereby facilitating polarization switching and significantly enhancing the direct piezoelectric response. Using this approach, we achieve a giant direct piezoelectric coefficient of $d_{33}\approx -271\mathrm{pC}/\mathrm{N}$ in relaxor ferroelectric polymers. This work uncovers the structural origin of the inefficacy of morphotropic phase boundary for enhancing direct $d_{33}$ in polyvinylidene fluoride-based polymers and demonstrates that substantial potential remains for improving their direct piezoelectric performance.

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• Crystal phenomena
• Phase transitions
• Dielectrics
• Ferroelectrics
• Piezoelectrics
• Polymer thin films
• Polarization
• First-principles calculations
• X-ray diffraction