The polymer network structures and topologies in hydrogels can significantly affect their mechanical properties. The network structures can vary dramatically with different synthetic conditions of hydrogels. Herein, we study the effects of the polymer network structures on mechanical properties of poly(acrylamide) (PAAm) hydrogels synthesized through free radical polymerizations. We focus on investigating how the initial monomer concentration ${\varphi }_0$ during the synthesis influences the network structures and thus their mechanical properties. Specifically, we measure the swelling capability of the polymer network, the elasticity and fracture of swollen hydrogels with a fixed volume fraction of polymer. Our results have revealed that for PAAm hydrogels with the same volume fraction of polymer, their elasticity, stretchability, work of rupture and fracture toughness can vary significantly if ${\varphi }_0$ during the synthesis is different. Our study may provide important guideline for synthesizing hydrogels with tailorable mechanical properties without changing the chemistry and synthesis procedure.

水凝胶中的聚合物网络结构和拓扑结构会显着影响其机械性能。网络结构可以随着水凝胶的不同合成条件而显着变化。在此，我们研究了聚合物网络结构对通过自由基聚合合成的聚（丙烯酰胺）（PAAm）水凝胶机械性能的影响。我们专注于研究初始单体浓度如何${\phi }_0$在合成过程中影响网络结构，从而影响它们的机械性能。具体而言，我们测量聚合物网络的溶胀能力、具有固定聚合物体积分数的溶胀水凝胶的弹性和断裂。我们的结果表明，对于具有相同聚合物体积分数的 PAAm 水凝胶，如果${\phi }_0$在合成过程中是不同的。我们的研究可能为在不改变化学和合成程序的情况下合成具有可定制机械性能的水凝胶提供重要指导。